Learn Swedish: Anmäler ni stulna bilar? - Are you plural reporting stolen cars?

Past participles work just like normal adjectives when you put them directly in front of a noun. Because bilar (cars) is a plural noun, the past participle stulen (stolen) takes the plural ending -a, becoming stulna.

What verb does stulna come from?

It comes from the irregular verb att stjäla (to steal). When turned into an adjective (a past participle), its base form is stulen for en-words, stulet for ett-words, and stulna for plural or definite nouns.

Why does the sentence start with the verb Anmäler?

In Swedish, yes/no questions are formed simply by putting the verb first. While English uses helper words (like 'Are you reporting?' or 'Do you report?'), Swedish just swaps the verb and the subject. The statement Ni anmäler (You report) becomes the question Anmäler ni? (Are you reporting?).

Is ni being used as a formal singular 'you' or a plural 'you' here?

In modern Swedish, ni is almost always the plural 'you' (referring to more than one person, like 'you guys' or 'y'all'). Using ni as a formal singular 'you' is very old-fashioned today, so you can safely assume it is plural in this sentence.